Conventionally, liquid electrolytes having a lithium salt dissolved in a non-aqueous solvent have been used as liquid electrolytes used in lithium secondary batteries. Furthermore, mixed solvents of ethylene carbonate, propylene carbonate, diethyl carbonates etc. are generally used as the non-aqueous solvent.
These carbonate-based solvents are generally used as the non-aqueous solvent, but suffer from a problem of poor oxidation resistance. Accordingly, there is a demand for hardly oxidized liquid electrolytes, from the viewpoint of improving the performance of lithium secondary batteries. Generally, liquid electrolytes are preferably those hardly undergoing oxidation and reduction; in order words, liquid electrolytes with a wide potential window are desired.
On the other hand, lithium secondary batteries having a BF3 complex added to a liquid electrolyte are known. For example, Patent Document 1 discloses a non-aqueous lithium battery using a BF3 complex as a capacity reduction rate-suppressing additive. Patent Document 1 addresses prevention of a lithium secondary battery from reducing its capacity caused during long-term use by using a BF3 complex as an additive. Patent Document 2 discloses a non-aqueous electrolyte secondary battery comprising a Werner-type complex of boron trifluoride. Patent Document 2 aims at preventing a film of lithium halide such as LiF from generating on the surface of an anode by using a BF3 complex as an additive, thereby suppressing an increase in battery impedance.
In both of Patent Documents 1 and 2, however, the BF3 complex is used absolutely as an additive, and the amount of the complex used is very small. Specifically, the amount of the BF3 complex is about 1 to 5% by weight based on the electrolyte in Patent Document 1, and the amount of the BF3 complex is about 0.5 to 5% by weight based on the whole of the liquid electrolyte in Patent Document 2. Furthermore, in Patent Documents 1 and 2, there is absolutely no description to the effect that the performance of the lithium secondary battery is improved by widening the potential window of the liquid electrolyte.
Patent Document 3 discloses an electrode active material for lithium secondary battery, which further comprises an amphoteric compound such as a BF3 complex in an electrode active material.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. H11-149943    Patent Document 2: JP-A No. 2000-138072    Patent Document 3: JP-A No. 2005-510017